Pyrophoric element



Aug. 6, 1957 Y c. c. BALKE ETAL 2,801,590

PYROPHORIC ELEMENT Filed Jung 14 L 195; s sheets-sheet 1 56 maire E Balke William 5. Graff Aug 5, 1957 c. c. BALKE ETAL 2,301,590

PYROPHORIC ELEMENT Blah- E Elalka BY William El. [1-'aff .Harney lll rrnornonrc ELEMENT Claire C. laike, Drexel Hill, and William S. Grad, Philadelphia, Pa., assigrors to the United States of America as represented by the Secretary of the Army Application .lune 14, 1951, Serial No. 231,596

Claims. (Cl. M12-9d) This invention relates to a new and improved pyrophor and to the equipment and process used in manufacturing it.

Pyrophors have been well known since the days of gas illumination when they were utilized in ints for lighting the gas. While almost any metal in the finely divided state exhibits pyrophoric properties, that is, oxidizes Vspontaneously when exposed to air, a few metals, among which cerium is probably the best known, .are of -such a nature that when abraded they yield a shower of hot sparks capable of igniting a hydrocarbon vapor. As a result cerium was widely used in the gas lighter llints. Today pyrophors are common in cigarette lighters and vin ilints for use in chemical laboratories. The composition of the modern pyrophor varies but usually includes some of the rare earth elements, misch-metal representing `a popular alloy with suitable sparkingsqualities.

Our invention .deals with pyrophors that can be used as incendiaries, .particularly for military purposes. Consequently the main object of the inventionis development of a pyrophor suitable as a military incendiary. Since the usual pyrophors are rather soft, theyare too weak to withstand the rough usage incident to militaryactivity. A second object of the invention 4is therefore provision of a pyrophor strong enough for military usage. We have found that suicient strength can 'be imparted to apyrophoric element by coating it with or encasingiitfiu a stronger material. A third object of the invention is the development of a pyrophor strengthened .by a coating of a non-prophoric material. A suitable formfor appyrophor meeting the requirements of our military incendiary is that of a rod coaxially coated with :strengtheningr and protective material. A 'fourth object of .our ,invention is consequently provision of a coaxial pyrophorby means of powder metallurgy. The coating can be'imparted to the powdered vpyrophoric material if both .are extruded simultaneously and concentrically from ia suitable die. Still another object of the invention` is provisionof a die for producing comparatively strong pyrophors by means of powder metallurgy.

Other objects of our invention willbe evidentfrom the following description and the `appended drawings fin which:

Figure l is a side elevation of the die of our invention;

Figure 2.is a vertical sectional View taken Lalongthe line 2-2 of Figure l;

Figure 3 is a cross-section taken alongthe line 3`3 of Figure 2;

Figure 4 -is a cross-section taken along line 44of Figure 2;

Figures 5 and 6 are sections taken along lines '5 5 and 6 6, respectively, ofFigure 2;

vFigure 7 is a cut-away view'ofone form .of the v.extruded pyrophor;

Figure is a ow sheet presenting our entire process; and

Figure 9 isa diagram presenting conditions duringpart of our process.

In general our pyrophoric element is a coaxial metallurgical composite with a length relatively great compared to its cross-sectional dimensions. It is composed of a core containing zirconium, such as a mixture of zirconium, titanium and lead which sparks intensely when abraded, and an enveloping ferrous case. The case imparts sufficient mechanical strength tothe pyrophoric core for the composite -to penetrate targets. The cross-sectional area of the core varies from 10% yto about 75% of the total, 15% being found particularly advantageous. Both core and case are made highly porous by the process Yof manufacture, porosity extending up 4to about V50% for the case and to about 40% or somewhat less vfor the core. This process consists essentially of coaxial and ,simultaneous extrusion of the core and Icase material-s as plasticized metallic powders in the cold state from a specially designed die adapted to give the proper shape and varea rto the desired product. An organic material such as synthetic rubber is utilized as a plasticizing agent. The plasticizer is decomposed after extrusion'byrsintering, leaving a porous core with incendiary properties and Ya case witha strong metallic structure.v Details of ,our invention will be found in the following description.

The first step in our process consists of K.mixing separately, the raw materials of the case and of the core. The .main constituent of the case is electrolytic iron powder. Since the porosity of the resultant productjispaninrportant feature, thepartcle Size of the powder must ,be .Controlled- Table I shows tolerable limits Von the particle sizeofthe .iron `powder determined by ,the Amesh .dimensions -9f column 1. Column 2 shows `the .limits of a particular vvmesh or mesh range while column v3 .shows a specific example found useful.

The ironpowder has a carboncontentof flessthan about 0.25% and an oxygen content of less .than about 1.0%.

Preferably nickel ,is employed as analloying agent Vfor .strengthening the case. The nickel is ,added as VNiVCVOs `powder in the vamount -of from .4 .to 11% of ,the total Fe-NiCO3 mixture. The upperlimit correspondsto about 5% by weight of nickel. The carbonateisblended,mechanically with the iron powder by a ball mill .orother means to disperse it as finely 'as possible.

There are many decomposable organic ,materials availablefor making the powders plasticand extrudable. Synthetic rubber known in the trade .as Perbunan or fParacril is preferably utilized however. The.plasticizer Vis added to the metallic powderin theform of a xylene solution or dispersion containing, for example, about .4% of the Perbunan in cc. of solvent. For optimum plasticity, the xylene-Perbunan solutionis added .tothe Fe-lNiCOa mixture in the ratio of about 1.5 cc. toa gram of mixture to give about 6% Perbunan by weight cal- -culated on the weight of iron.

The preferred pyrophoric powder, as stated `abov,e,consists of the three elements zirconium, titanium and lead. A particularly advantageous composition contains 35% Zr, 30% Ti and 35% Pb. The preferred Zr and Tipowders should be finer than l5 microns while the preferred Pb is -200 mesh. A xylenePerbunan solution of the lsamepercentage as that used above is added vto the pyrophoric -blend in the ratio `of about 0.875 cc. of solution to a` gram of powder. This ratio yields 3.51% {Perbunanby weight `based on the weight of powder y audfis ,calculated L3 to provide sufficient plasticity for extrusion upon the evaporation of the solvent.

Tables 2 and 3 give the raw material limits for case and core respectively together with, in column 3, a specific example. The limits of 500 g. Fe to 75 g. pyrophoric mixture are fixed by the design of the coaxial extrusion die yielding a core area equal to 15% of the case area. It will be understood however that with other relationships between the core and the case areas within the range set forth above, the ratio of iron to pyrophoric powder can be varied to meet the requirements of the die.

Table II--Case material Zr (-15 mierons) Ti (15 microns). Pb (-200 mesh)- 2.625g (3 56.25-92.75 ce .5%). 65.6 ce. (4% Perbunan).

Xylene Solution After the Perbunan solutions have been added to their respective powders, the mixtures are stirred to form more or less homogeneous slurries and then evaporated separately, with constant stirring, on steam baths. Evaporationis continued until the odor of Xylene has been greatly reduced and each slurry is plastic and dough-like. In this condition the slurries are pre-extruded under about 1/2 to 3 tons total pressure through a 1A" diameter orifice in an extrusion die. After this first or pre-extrusion each mass is a homogeneous blend of plasticizer and metal powder.

When the plasticized masses comprising the case and core materials have been passed through the pre-extrusion die they are inserted into the appropriate receptacles of the especially designed coaxial die of our invention. This coaxial die, described below, permits the simultaneous extrusion of case and core at the same rate of speed at their point of juncture with an extrusion ratio greater than twenty, preferably about thirty. The extrusion should take place at a total pressure of less than about ten tons` and should proceed very slowly to yield the strongest possible green extrusion. The rod-like length of pyrophoric element obtained may then be coiled or otherwise shaped prior to drying in an oven at 80 C. for several hours to evaporate the residual xylene. After drying the element still retains sufficient flexibility to be handled and transported to a sintering furnace.

The essential feature of our novel coaxial extrusion die can be seen by reference to the drawings. The plasticized case material is inserted into an outer barrel 20 concentric with an inner barrel 22 serving as a container for the plasticized core material. Pressure is applied to both materials simultaneously by plungers 25. and 26 interconnected externally by platen 28. The outer barrel 20 terminates at a spider 30 which supports and aligns both barrels. Holes 32 in spider 30 join the hollow interior of barrel 20 with the outer portion 3d of an intermediate `orifice 38 formed within orifice retainer 40. The inner portion 36 of the intermediate orifice 38 is the exit end of inner barrel 22. Figure 5 shows the arrangement of outlets at the intermediate orifice. Passage 42 connects the intermediate orifice 33 through tube 44 and second orifice retainer 46 to the final orifice 48. Jacket 50 and supporting rings 52 and 54, held together by screws 56, complete the die structure. At the intermediate orifice 38 the core and case materials under extrusion join coaxially at the same rate of speed, the

area of the core being fixed at 15% of the case area. The ratio of barrel area to orifice area is 30 at the intermediate orif'ice. The element is further shaped and reduced 20% by the final orifice 43. ln the die shown the barrels and intermediate orifice are of round cross-section while the final orifice (Figure 6) develops the coaxial element into a square cross-section. This element is shown in Figure 7 where the core 60 is surrounded by the square case 58. Other shapes can be developed by changing the design of the final orifice, thus producing a circular, hexagonal, ribbed or other coaxial element as desired.

The basic features of the co-extrusion die described ab ove are as follows:

l. The plasticized core material is contained in an inner barrel and the plasticized case material is contained in a separate outer barrel concentric with the inner one;

2. Both barrels connect with concentric intermediate orifices in such a manner that the core and case materials are brought together at the same speed at one point, there, being a pre-calculated ratio between core area and case area at this point;

3. There should be a pre-calculated reduction from barrel areas to intermediate orifice area to permit compaction of both components and to impart sufficient strength for handling after extrusion. The ratio of barrel cross-sectional area to intermediate orifice area should be greater than twenty;

4. The intermediate orifices connect with `a final orifice that develops the cross-sectional shape and dimensions of the nal extrusion and provides for a slight final reduction in area of the coaxial shape to bring the core and case into more intimate contact. The ratio of the total intermediate orifice area to final orice area should be greater than one but less than two so that the ratio of core area to case area will not be greatly altered;

5. The core and case materials in their respective barrels are acted upon by two plungers. rPhese plungers can move at the same rate of speed because the two materials extruded have the same reduction in area as they pass from their respective barrels through the intermediate orifices. In other words, the extrusion ratios of both components are the same.

After extrusion from the novel compound die the coaxial element is removed to an oven and dried to evaporate the remaining xylene. The element is then ready for a sintering operation to remove the organic plasticizer. The essential features of the sintering process involve Ia sequence of atmospheres of vacuum, hydro` gen and vacuum applied with temperatures ranging up to 1400 C. This process is carried out in an oven in three distinct periods corresponding to the intervals during which each of the above named atmospheres is utilized. Figure 9 shows diagrammatically the sintering s-chedule. The first period involves a vacuum of less than 1000 microns until the sintering temperature is reached. Period 2 requires hydrogen either at partial or atmospheric pressure. Period 3 is a second vacuum heat treatment which should finish at a pressure of less than microns. The entire sintering operation is followed by a final cooling period in vacuo. in detail the sintering process is as follows:

1. The first period of the sintering operation is carried out in a vacuum to remove the major portion of the plasticizer by destructive distillation and prevent absorption of gases by the Zr, a powerful getter metal tending to form non-pyrophoric compounds. 'Ihe particular synthetic rubber plasticizer referred to above is distilled off at 20G-600 C. Here the vacuum pumps must pull the gases out rapidly and minimize the amount of carbon residue. Such a residue forms zirconium carbide reducing the pyrophoric properties of the core. The rate of ternperature rise in this region is limited by the rate of distillation of the plasticizer and must be slow enough so that the pumps can keep the pressure under 1000 microns.

Once the distillation is over the temperature may be raised to the desired top temperature. Period 1 is represented by interval A of Figure 9;

2. The second period of the sintering operation is carcied out in atmosphere produced by bleeding hydrogen into the furnace to maintain a pressure greater than 1000 microns or by shutting off the vacuum system and flowing hydrogen through the furnace at atmospheric pressure. The latter method is more practicable and also yields a more strongly reducing atmosphere for the reduction of iron oxides, forming a dense and strong ferrous case. At the top sintering temperatures of around 1000 C., Zr absorbs little hydrogenand most of that absorbed may be removed by a subsequent vacuum treatment at these temperatures without impairing the pyrophoric properties of the core. The time in hydrogen, interval B of Figure 9, should be at least 3A hour;

3. The third period, interval C of Figure 9, of the sintering operation is again carried out in a vacuum. The charge is kept heated until the vacuum reaches 100 microns or less. The vacuum is continued until the Zr core material has been outgassed and has become well sintered. The element is cooled in a vacuum since otherwise Zr would absorb large quantities of gas. After cooling the incendiary element can be removed from the furnace ready for use.

The times involved in all these three periods depend entirely upon the mass of material to be sintered and the pumping capacity of the vacuum system, and are not an essential feature of the disclosure. The resulting final element has however suicient strength and ductility to be further cold-worked by hammering or rolling and sintered a second time, if desired to reduce porosity and increase strength. It is suciently strong to withstand explosive shocks and is very durable as far as oxidation or deterioration is concerned.

S0 far in this specification the process we have developed for the manufacture of pyrophors by the methods of powder metallurgy has been described in some detail. The main outlines of the process ars shown in the flow diagram of Figure 8. This figure is believed self-evident. By way of illustration of the entire process the following example is given:

500 g. of 200 mesh electrolytic Fe powder, containing less than 0.25% carbon and less than 1.0% oxygen and having a mesh fraction of 14.5% -200-1-270, 13.5% -270}325, and 72.0% 325 mesh, was blended with 24.75 g. of NiCOs for one hour in a ball mill. This blend was mixed with 750 cc. of xylene solution containing 30 g. of a synthetic rubber known as Perbunan. The slurry was evaporated for about two hours on a steam bath until the mass had a putty-like consistency. It was then extruded through a 1A" dia. orifice in a die having a 1 dia. barrel under a total load of about 11/2 tons to give a pre-extruded rod. Preextrusion was repeated several times in order to give the mass as uniform a consistency as possible.

A mixture of Zr, Ti, and Pb powders totaling 75 g. and comprising 35% Zr, 30% Ti, and 35% Pb, all having a particle size of less than microns with the exception of the Pb which was -200 mesh, was mixed with 65.6 cc. of the xylene solution containing 2.625 g. Perbunan. The slurry was evaporated on a steam bath in a manner similar to that described above. The mass was pre-extruded through the 1A orilice to obtain a uniform texture.

The 500 g. mass of the Fe material was placed in the outer barrel of the co-extrusion die and the 75 g. mass of Zr material was placed in the inner barrel. These two weights were in the right proportion so that co-extrusion would start at once when the plungers contacted the masses. The total pressure on the co-extrusion die amounted to about 2 tons and extrusion proceeded slowly to give a smooth surface and strong green strength. The compacted material could still be wound on a 3 dia. mandrel in the form of a helix with a pitch of 1A" per turn. The 1A square element has a core area equal to about 15% of the area of the case. The mandrel with the helix was then dried in an oven at C. for three hours to remove the residual xylene after which time it was mounted on a saddle for placement in the sintering furnace.

In the case of the example, heating was carried out by means of high frequency. The furnace consisted of a closed-end silica tube containing a sheet iron secondary for the heating element. The open end of the silica tube was connected to a series of vacuum pumps with a freezing trap interposed between furnace and pumps. A vacuum of 20 microns was drawn on the cold furnace at which time heating was started. The temperature was slowly raised at such a rate that the vacuum did not drop below 1000 microns and care was taken in passing through the 200 to 600 C. range where most of the Perbunan was distilled out of the extrusion. The temperature was then raised more rapidly until a top temperature of 1100 C. was reached. The total time required to reach this point was 11/2 hours. Hydrogen was then admitted at atmospheric pressure, after purification, and the 1100 C. temperature was continued for 45 minutes. The hydrogen was then turned olf and the Vacuum pumps turned on. Vacuum sintering was continued for 1% of an hour longer until a vacuum of less than microns was obtained. Then the furnace was disconnected and the material allowed to cool to below 200 C. before withdrawal into the air.

The sample helix was approximately 6 long by 3" I. D. by 31/2" O. D. and showed a good uniform gray surface with no warping. The Fe was sufficiently ductile to withstand hammering to as much as 25% reduction. When abraded the pyrophoric core sparked intensively.

The foregoing example was given solely by way of illustration. Other modifications will be apparent to those skilled in the art. r In fact, we wish to be bound solely by the scope of the appended claims.

What we claim is:

l. An incendiary comprising a sintered porous pyrophoric zirconium alloy encased in a strengthening and protecting porous ferrous material.

2. An incendiary consisting of a sintered porous rod of a pyrophoric zirconium mixture strengthened and protected by an integral enveloping porous ferrous case.

3. An incendiary consisting of a sintered porous rod, made from a pyrophoric mixture of zirconium, titanium and lead, strengthened and protected by an integral enveloping porous ferrous case.

4. An incendiary consisting of a sintered porous rod, made from a pyrophoric mixture containing about 35 by weight of zirconium, about 30% by weight of titanium and about 35% by weight of lead, strengthened and protected by an enveloping porous ferrous case integral therewith.

5. As an article of manufacture, an elongated metallic rod consisting of a pyrophoric core encased in a porous ferrous envelope integral therewith, the pyrophoric core being formed from a zirconium alloy containing about; 35% by weight of zirconium, about 30% by Weight of titanium and about 35% by weight of lead.

References Cited in the le of this patent UNITED STATES PATENTS 1,184,753 Keplinger May 30, 1916 1,984,775 Swartz Dec. 18, 1934 2,073,465 Deitz Mar. 9, 1937 2,097,502 Southgate Nov. 2, 1937 2,225,424 Schwarzkopt Dec. 17, 1940 2,323,303 Bluehdorn July 6, 1943 2,409,307 Patch Oct. 15, 1946 2,417,437 Nicholas Mar. 18, 1947 2,490,570 Anicetti Dec. 6, 1949 2,532,323 Miller Dec. 5, 1950 2,611,316 Alexander Sept. 23, 1952 

1. AN INCENDIARY COMPRISING A SINTERED POROUS PYROPHORIC ZIRCONIUM ALLOY ENCASED IN A STRENGTHENING AND PROTESTING POROUS FERROUS MATERIAL. 